Large dump trucks, usually eighteen-wheelers, having a tractor and a trailer are used to haul and dump large quantities of material, such as dirt, gravel, etc. The rear portion of the trailer nearest the back wheels may or may not be rigidly mounted on a chassis frame. On a frameless truck where the trailer is not rigidly mounted to the wheel chassis, the tandem rear wheels on either side towards the front of the bed are also lifted from the ground when the trailer is raised, so that the points of contact between the trailer and the earth are only on the very rear tandem wheels. When dumping a load, the front end of the trailer nearest the tractor is raised by means of an hydraulic telescoping cylinder, and the dumped material exits the trailer through a hinged tailgate at the rear of the trailer. Because of the nature of the location where these trucks are often used, the ground surface when loading and unloading the dump truck is usually not level; and if the load is dumped on a fresh land fill, the underlying ground may settle rapidly because of the weight of the truck, and/or the load within the trailer may shift. When the truck trailer is raised for dumping the transported material, the center of gravity of the truck trailer is also raised, and if the magnitude of the lateral force vector at the center of gravity exceeds the magnitude of vertical force vector, such as when the truck is on an uneven surface or when the load shifts suddenly, the truck trailer tips over. If the trailer tips, the transported material may be damaged, the truck trailer and tractor may be severely damaged, and worst of all, human life may be harmed or lost.
Truck drivers, truck owners and construction site operators are not unaware of the possibility of a truck trailer tipping over and the resultant loss of time, materials, labor and possible human life. As a result, a number of devices have been disclosed which may level the trailer of the truck, or indicate that an unsafe condition exists when the trailer is unstable, or may even automatically lower the trailer if a hazardous condition exists. Such devices for detecting when a truck trailer is approaching a tip-over condition are generally placed on the underside of the trailer bed or adjacent to the rear wheels, and measure the deviation of the roll angle of the truck bed from the horizontal. Examples of this type of device are given U.S. Pat. No. 4,682,145, entitled "TRUCK LEVEL SENSING AND INDICATING SYSTEM" to Brawner, Jr.; U.S. Pat. No. 4,216,996, entitled "STABILIZER FOR DUMPING VEHICLES" to Pitts; U.S. Pat. No. 3,921,128, entitled "TRUCK ROLL WARNING SYSTEM" to Snead; U.S. Pat. No. 3,640,578, entitled "LEVELLING SYSTEM FOR SEMI END DUMP TRUCKS" to Finney; U.S. Pat. No. 3,464,755, entitled "DUMP TRAILER SAFETY DEVICE" to Brown.
Brawner, Jr. '145 employs a plumb vane mechanism mounted on the bottom of the trailer frame to measure both the pitch and roll axis of the trailer, and when the roll angle exceeds certain preset limits a visual signal is generated. Pitts '996 teaches a stabilizer system also mounted under the trailer bed and adjacent to the rear wheels of the trailer which incorporates a pendulum means oriented to swing transversely to the chassis. Thus, the Pitts '996 invention measures the roll angle of the trailer with respect to the vertical axis. When the pendulum swings out of the vertical plane and contacts one of two microlimit switches on either side, a hydraulic system is triggered to level the trailer. Snead '128 teaches the use of two mercury switches mounted at the center underside of the dump bed; each switch responds when the roll angle of the trailer exceeds a predetermined angle in either direction and an alarm is triggered and a safety valve is opened to lower the truck trailer. The Snead '128 invention compensates for the vertical angle or the pitch of the trailer bed and teaches that the critical roll angle of the dump bed is different when the trailer bed is lowered than when the trailer bed is raised. Finney '578 likewise measures the roll axis of the trailer bed using mercury switches located along the center bottom of the trailer.
There are several disadvantages associated with the devices presented in the prior art that are overcome by the present invention. One disadvantage is that the measuring devices of all of the above inventions are placed towards or at the rear of, and mounted on the underside of the truck trailer. Access to the devices for repair and maintenance is difficult and can be hazardous. Secondly, the type of device actually used to detect the roll angle of truck trailer disclosed in the prior art also presents a severe disadvantage overcome by the present invention. Except for the pendulum means of Pitts '996 and the plumb vane means of Brawner, Jr. '145, all referenced prior art devices incorporate mercury switches to indicate when the roll angle of the trailer bed exceeds a predetermined safety limit. Mercury switches, under these circumstances, are extremely sensitive to small displacements, and a displacement of one side of the trailer bed of only three or four inches with respect to the other side of the trailer bed will cause the mercury switch to detect a "hazardous" condition. For instance, when a traveling truck hits a pot hole or is on an uneven surface, such as an unimproved rocky road, one side of the truck trailer could easily be displaced a distance of three or four inches relative to the other side of the truck trailer, and the alarm systems disclosed in the prior art would be inadvertently triggered. This occurs frequently enough so that the truck operators either turn off or permanently disconnect the detection and warning system to avoid the continuous aggravation of these false alarms.
Yet, another crucial disadvantage associated with the prior art, and as previously mentioned, is that all of these devices taught in the prior art measure the roll angle of the trailer, i.e., the rotation of the trailer bed about the center axis extending the length of the trailer. But, when the trailer is raised, the actual angle which determines whether the trailer will tip on its side is not the angle which the trailer rotates on its own axis (the roll angle), but the degree angle peading through which the front end of the trailer travels along a radial path having a radius determined by the length of the telescoping hydraulic cylinder with the pivot at the fifth wheel. The present invention incorporates an angle measuring device mounted on or near the fifth wheel of the trailer which measures the angle of revolution (tip angle) of the front end of the extended trailer about the fifth wheel pivot. Further, the tip angle is measured at the most distant point from the fifth wheel, i.e., directly above the fifth wheel, when the telescoping hydraulic cylinder is extended. Assuming, for the sake of geometrical illustration, that a point on the fifth wheel of the trailer is labelled Point A, and directly above Point A on the bottom of the raised trailer is Point B. Also assume, that through Point B, perpendicular to Line AB, is a line extending equidistant on either side of Point B, whose end points are Points C and D. Line CD represents the width of the bottom of the trailer bed, approximate dimensions being ninety to one hundred two inches, and Point B represents the center of the trailer bed at the front end of the trailer. The prior art referenced herein measures the rotation of a line, which is parallel to Line CD about the roll axis of the trailer bed. Therefore, using the suggested critical angle of five degrees as taught in Snead '128, a displacement of the side of the truck with respect to Point B or the other side of the bed of approximately 3.9 to 4.5 inches would indicate a dangerous situation in the roll of the trailer bed. Brawner, Jr. '145 measures the angle from which one of the rear wheels deviates from the other rear wheel on the opposite side of the trailer. A displacement of approximately three inches indicates that roll angle of the trailer bed is approaching a dangerous angle. Referring to Points A and B as described above, the present invention, as embodied herein, measures the rotation of Point B about Point A, or the rotation of the trailer bed around the fifth wheel with the hydraulic telescoping cylinder as the radius arm. Using actual measurements, the dimensions of an extended hydraulic telescoping cylinder used on a dump truck is typically two hundred ninety six to four hundred inches, and a determination of the critical tip angle at which a warning system should indicate a hazardous condition is approximately two and one-half to four degrees. The distance through which the raised trailer as to move to trigger the warning and alarm system described herein is given by the tangent of the critical tip angle (21/2.degree. to 4.degree.) multiplied by the dimensions of the extended hydraulic cylinder (296 to 400 inches). Therefore, a movement on Point B approximately thirteen to twenty-nine inches of either side of vertical will be adequate to trigger the warning system embodied herein depending on the dimensions of the hydraulic cylinder and the value of the critical angle used. This difference between measuring approximately three or four inches at the rear of the trailer (as taught by the prior art) to the measurement of approximately thirteen to twenty-nine inches (at the front of the trailer, by the use of a taut cable between Points A and B, which cable is coupled to a pendulum having a pivot point at or near Point A) diminishes, if not eliminates, the possibility of false alarms.